Topic: Enthalpy and Table I Do Now: packet p.1

Chemistry is the study of matter and energy. So far we have learned about the types of matter and how they can physically combined to make mixutures or chemically combine in chemical reactions to from new compounds. Every physical or chemical change is accompanied by energy change  Energy released = _________________  Energy absorbed = _______________ exothermic endothermic GLSGLS exothermic endothermic The universe wouldn’t exist without energy. It provides all our work, our heat, can be stored to use later and it never goes away (conservation of energy) The total amount of energy a substance contains depends on many factors, some of which are not totally understood…it’s impossible to know the total heat content of a substance. So scientists measure scientists can measure the amount of heat lost or gain by a system

H = Enthalpy = heat ΔH = Heat of Reaction The heat gained or lost by the system (at constant pressure) ΔH = H products – H reactants ΔH=Q Energy is stored in chemical bonds

For any reaction occurring at constant pressure ΔH = Q Q = mC  TQ = mC  T –Q = Energy change –m = mass of water –C = specific heat of water –  T = temperature change = T f – T i 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s) kJ OR 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s)  H = kJ (heat is lost) NH 4 NO 3 (s) + 27 kJ  NH 4 + (aq) + NO 3 - (aq) OR NH 4 NO 3 (s)  NH 4 + (aq) + NO 3 - (aq)  H = 27 kJ (heat is gained)

If ΔH is negative… - ΔH = exothermic – heat is lost -H products < H reactants -PE of Products < PE of Reactant -Example: 4Fe (s) + 3O 2(g)  2Fe 2 O 3(s) kJ -ΔH = -1625kJ

If ΔH is positive… + ΔH = endothermic – heat is gained -H products > H reactants -PE of Products > PE of Reactant Example: NH 4 NO 3 (s) + 27 kJ  NH 4 + (aq) + NO 3 - (aq) -ΔH = +27kJ

Universe Environment System A B Since energy is conversed…the system changes in one direction and the surrounding have to change in the opposite direction A. Reaction is Exothermic, environment gets _________________ B. Reaction is Endothermic, environment gets _________________ warmer colder

Since there are different types of reactions, you have various ΔH’s ΔH comb = enthalpy (heat) of combustion – the enthalpy change for the complete bunring of one mole of the substance ΔH formation = enthalpy (heat) of formation – the enthalpy change for the formation of a compound from its constituent elements ΔH solution = enthalpy (heat) of solution – the enthalpy change when 1 mole of an ionic substance is dissolved in water. Look at Table I: Heats of Reaction Lets label the various types from above

Rxns 1-6: combustion rxnsRxns 1-6: combustion rxns   H = heat of combustion Rxns 7-18: formation (synthesis) rxnsRxns 7-18: formation (synthesis) rxns –Substance is formed from its elements –  H = heat of formation Rxns 19-24: dissolving equationsRxns 19-24: dissolving equations –  H = heat of solution Table I

Many other processes other than chemical reactions absorb or release energy like, Changes of state  H vaporization = molar heat of vaporization = amount of heat required to vaporize one mole of a liquid  H fusion = molar heat of fusion = amount of heat required to melt one moles of a solid

Energy depends on amount Remember – it takes more energy to heat up water in the ocean than to make a cup of tea

CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O (l)  H = kJ 1 mole of methane + 2 mole of oxygen → 1 mole of carbon dioxide gas & 2 moles of liquid water reaction is ____________ (negative sign for ΔH) kJ energy released per mole of CH 4 (g) burned exothermic What would happen if we had 2 moles of methane? Twice as much energy would be released 2 x kJ = kJ will be released

Reactions: Energy depends on direction too! N 2 (g) + 3H 2 (g)  2NH 3 (g)  H = kJ 2NH 3 (g)  N 2 (g) + 3H 2 (g)  H = _______ If reverse equation, reverse sign of  H 91.8 kJ

Table I Exothermic: ΔH = - Pay attention to number of moles, may need to divide or multiply ΔH Pay attention to direction, if in reverse direction, switch the sign

Check POINT 1. According to Ref. Table I, the decomposition of which compound is exothermic? A) C 2 H 6 B)NO C) CO 2 D) NH 3 2. According to Ref. table I, what is the heat of reaction for the formation 1 mole of H 2 O (l) from hydrogen and oxygen gas at STP? A) kJ B) kJ C)-55.8kJ D) kJ

If we can’t calc. H for one individual substance how are we able to calc. ΔH???!

FYI: Hess’s Law (not on regents) Can add 2 or more equations by adding the  H’s Enables you to calculate  H for # of rxns Say you’re interested in 2S(s) + 3O 2 (g)  2SO 3 (g)

Have  H’s for the following: a) S(s) + O 2 (g)  SO 2 (g)  H = -297 kJ b) 2SO 3 (g)  2SO 2 (g) + O 2 (g)  H = 198 kJ 222 2SO 2 (g) + O 2 (g)  2SO 3 (g)   H = -198 kJ  H = -594KJ + (-  kJ   H = -792 kJ x (2)